Method of controlling multi-casing variable speed compressors

ABSTRACT

A method of controlling multi-casing variable speed compressors comprising a low pressure compressor and a high pressure compressor at least one of which is of axial type and has stator blades which are adjustable in setting angle, said method includes steps of measuring the pressure rise in one compressor and the total pressure rise in both said high pressure and low pressure compressors, adjusting the setting angle of the stator blades in accordance with the ratio of the pressure rise in said one compressor to the total pressure rise so as to obtain a predetermined share of pressure rise in said one compressor, and controlling the speed of the compressors in accordance with the output pressure or air flow in the compressor.

United States Patent 1 Shirato et al.

[ Oct. 23, 1973 [75] inventors: Takeshi Shirato; Toshiyuki Ogishi,

both of Okayama, Japan [73] Assignee: Mitsui Shipbuilding andEngineering Co. Ltd., Tokyo, Japan 221 Filed: Mar. 13, 1972 211 Appl.No.: 234,233

Schlirf 415/1 Berry 415/1 Primary Examiner-C. J. Husar Att0rneyl(arl W.Flocks [57] ABSTRACT A method of controlling multi-casin'g variablespeed compressors comprising a low pressure compressor and a highpressure compressor at least one of which is of axial type and hasstator blades which are adjustable in setting angle, said methodincludes steps of measuring the pressure rise in one compressor and thetotal pressure rise in both said high pressure and low pressurecompressors, adjusting the setting angle of the stator blades inaccordance with the ratio of the pressure rise in said one compressor tothe total pressure rise so as to obtain a predetermined share ofpressure rise in said one compressor, and controllingthe speed of thecompressors in accordance with the output pressure or air flow in thecompressor.

6 Claims, 2 Drawing Figures METHOD OF CONTROLLING MULTI-CASING VARIABLESPEED COMPRESSORS The present invention relates to a method ofcontrolling multi-casing variable speed compressors and moreparticularly to a method for controlling pressure ratio amongcompressors, that is, pressure distribution or load distribution betweenlow pressure and high pressure compressors.

In a conventional compressor having an intermediate cooler, pressuredistribution is not controlled. In an actual practice, a compressor isoperated either without any control or under a constant pressure controlin which output pressure is maintained substantially constant or under aconstant flow control in which air flow is maintained substnatiallyconstant. ln either case, any substantially is not performed forobtaining predetermined distribution of pressure among a low pressureand a high pressure compressors. Therefore, when the performance of ahigh pressure compressor is deteriorated, the pressure increase or theshare of load on the high pressure compressor is reduced and that on thelow pressure compressor is correspondingly increased. From theview-point of aerodynamics, this means a reduction in angle of attack inthe high pressure compressor and increase 'in the low pressurecompressor, with the result that the low pressure compressor is possiblyoperatedin a surging zone. This may ultimately cause a failure of thelow pressure compressor. When the low pressure compressor is providedwith an anti-surge control means which serves to prevent the surgingphenomenon by sensing the flow and the pressure in the low pressurecompressor, it may be possible to prevent the surging, however, when acontrol is made in such a manner that a constant air flow is maintained,there may be a substantial decrease in the outlet air pressure to such adegree that a plant operation may become impossible.

The deterioration in the performance of the high pressure compressor iscaused by dust carried by the air and absorbed in drain water separatedin the intermediate cooler and adhered on compressor blades, or byscales or the like produced by sulphuric acid corrosion or otherchemical corrosion of components of the intermediate cooler. Therefore,this deterioration in performance of the high pressure compressor cannotbe avoided as far as the intermediate cooler is used. Further, even ifthere is no adverse effect of the dusts or the like from theintermediate cooler, there inevitably occurs an unbalance in pressuredistribution among the compressors when it is operated at off-design.Thus, the compressor must be operated within a very limited range.

The present invention has an object to eliminate the aforementioneddisadvantages of the conventional methods.

To attain this, it is proposed to present a load distribution in a formof load shared by the low pressure compressor or the high pressurecompressor to the total load. In other words, the pressure ratio sharedby the low pressure compressor or the high pressure compressor ispreviously set to a optimum value as a function of the total pressureratio. The actual pressure ratio shared with the low pressurecompressoror the high pressure compressoris compared vwith the set ratio ina;controller and equalized by controlling the setting angle of thestator blade.

Another object of the present invention is to provide a method forcontrolling multi-casing compressors in which an unbalance in pressuredistribution between a high pressure and a low pressure compressor dueto a deterioration in performance of the high pressure compressor isself-compensated by controlling the pressure distribution so that thereis no decrease in capacity.

A further object of the present invention is to provide a method forcontrolling multi-casing compressors which is effective to prevent anyfailure of low pressure compressor blades which may be caused by anunbalance in pressure distribution between a high pressure and lowpressure compressors and which can provide sufficient safety margins tosurging points of the high and low pressure compressors when thecompressors are operated under a part load pressure being different froma design pressure.

According to the method of the present invention, it is possible toensure a safety of operation under a constant air flow and to maintain awhole pressure level and an air flow level through a control of therotational speed.

The method in accordance with the present invention is characterized bythe fact that the outlet pressure of the high pressure compressor or theultimate pressure and the air flow are controlled by regulating thespeeds of the high pressure and the low pressure compressors and theunbalance in pressure ratio is self-compensated by adjusting the angleof stator blades in the high pressure compressor. Thus, the low pressurecompressor can be of a fixed stator axial or centrifugal type which doesnot have any control means. Of course, it is also possible to make thestator blades of the low pressure compressor adjustable in lieu of thosein the high pres sure compressor. In this case, the load'distribution iscompensated in the low pressure compressor, and the decrease in totalcapacity can be recovered by an increase in speed.

The above and other objects and features of the present invention willbecome apparent from the following descriptions of a preferredembodiment which will be proceeded taking reference to the accompanyingdrawings, in which;

FIG. 1 is a diagrammatical illustration of one embodiment of the presentinvention; and,

FIG. 2 is a diagram showing the characteristic curves of a compressorwhich is controlled in accordance with the present invention.

Referring to the drawings, particularly to FIG. 1, the reference numeral(6) designates an inlet filterthrough which f'luidis sucked into a lowpressure compressor (2) which is driven by a variable speed prime mover1 (1) such 'as a steam turbine, a gas turbine or the like. The fluidcompressed in the low pressure compressor (2) is then cooled by aninterm'ediate cooler' (7). The atmospheric air usually includes watermoisture and when the air is cooled this moisture is made to condensate.The condensate has to be deleted from the pipe line as much as possible.At this instance, a condensate is usually separated from the-fluid andtherelative humidity of the fluid becomes percent. Even when the fluidis not cooled to the relative humidity of 100 percent, the portion ofthe fluid which is in contact with walls of cooling pipes is locallysupercooled until a condensate is separated therefrom. Although thearrangement is such that the drain is expected to be discharged throughsuitable means such as a drain trap, a portion of the drain is takeninto a high pressure compressor (3) in the form of a mist which mayadhere to blade surfaces to cause erosion thereof. The fluid introducedinto the high pressure compressor is expanded at the first blade rowtherein so that the temperature thereof is decreased. As the fluid is ofsuch a high humidity as 100 percent or so, a condensate is separatedtherefrom and adheres to blades in the next row. When the drain isdeposited on the blade surfaces, dusts are absorbed thereby resulting ina decrease in performance of the compressor. When the air includesulphur dioxide, a sulphate solution is produced by reaction with waterin the intermediate cooler and causes corrosion of the cooler parts toproduce scales of such as copper sulphate and iron sulphate which arealso adhered to the blade surfaces. A similar phenomenon may be produced even in an atmosphere including nitric acid or the like. It may bepossible to prevent these scales from being produced by selectingsuitable materials, however, in any event, a small quantity of scaleswill be produced.

From the above descriptions, it will be apparent that contamination ofcompressor blade will cause a deterioration in performance of the highpressure compressor resulting in a decrease in capacity and the range inwhich the compressor can be operated without surging. In order toprevent such disadvantage, it is only necessary in the characteristiccurve of FIG. 2 wherein the axis of ordinate represents the load ratioL(%) and the axis of abscissa the flow ratio F(%), to shift thecharacteristic curve B of the high pressure compressor after thecontamination to the position of a selfcompensated characteristic curveA, by varying the setting angle of the stator blades of the highpressure compressor. In a strict sense, the characteristics A' does notexactly coincide with the characteristics A which is obtained by a newcompressor blades, however, it is practically acceptable because bothcharacteristics substantially coincide with each other in the actualoperating range. I

Now, referring to the control in accordance with the present invention,in case of a constant flow control, a pressure signal from a pressuresignal generator is introduced into a pressure controller which producesan output for controlling a rotational speed controller (16). When achange-over switch (18) is selected to control the flow, a signalfromthe flow detector (11) which is attached to a venturi or an orifice isfed to a flow controller (17) and compared with a set signal, thuscontrolling the speed controller 16) until any difference between bothsignals is completely eliminated. Thus, a constant air flow can bemaintained. The pressure ratio between the high pressure and the lowpressure compressors is controlled in the following manner. Pressuresignals from an inlet pressure signal generator (8) and an outputpressure signal generator (9) in the low pressure compressor areintroduced into a pressure ratio operator (12) which produces a pressureratio signal for the low pressure compressor. Further, pressure signalsfrom the inlet pressure signal generator (8) of the low pressurecompressor and from an outlet pressure signal generator (10) of the highpressure compressor are introduced into a pressure ratio operator (13)which produces a total pressure ratio signal corresponding to thatbetween the inlet pressure of the low pressure compressor and the outletpressure of the high pressure compressor. The total pressure ratiosignal is introduced into a function generator (19) which determines theratio of the pressure in the low pressure compressor in accordance witha predetermined curve throughout the operating range of the compressor.The output of the function generator (19) is then introduced into apressure ratio controller (14) and compared with an actual pressureratio signal in the low pressure compressor to adjust the stator blades(4) in the high pressure compressor by means of an actuator (5) inaccordance with the amount of difference between the two signals.

In accordance with the present invention, a pneumatic pressure system,an electro-hydraulic system or other suitable system may be used forperforming the control. When the inlet pressure of the low pressurecompressor is substantially constant, a similar result can be obtainedby using the output pressure signal generators (9) and (10) in lieu ofthe pressure ratio operators (l2) and (13), respectively. Further, asimilar result can be obtained by adjusting the stator blades in the lowpressure compressor, although the detail of the operation is notdescribed herein.

From the above descriptions, it will be understood that in accordancewith the present invention the pressure ratio can be controlled byadjusting the setting angle of the stator blades of the compressor, sothat the following advantages can be obtained.

1. The compressor is free from decrease in capacity.

2. Both the low pressure and high pressure compressor have substantiallythe same amount of safety margins of surging, so that the compressor canbe operated substantially free from surging.

3. Even under a part load condition in which pressure and air flow aredifferent from a design value, it is possible to perform a pressureratio control so that an increased range of operation can be obtainedunder various rotational speeds and at the same time the compressor canhave a wider high efficiency range.

We claim:

1. A method of controlling multi-casing variable speed compressors atleast one of which is of axial compressor type and has stator bladeswhich are adjustable in setting angle, said method comprising steps ofdetermining the unbalance in pressure distribution, adjusting thesetting angle of the stator blades so as to compensate unbalance inpressure distribution between the compressors, and controlling the speedof the compressors for adjusting the total pressure rise or air flow.

2. A method in accordance with claim 1 in which said compressors includea high pressure compressor and a low pressure compressor, said highpressure compressor having adjustable stator blades.

3. A method in accordance with claim 1 in which said compressors includea high pressure compressor state and a low pressure compressor stage,said low pressure compressor having adjustable stator blades.

4. A method in accordance with claim 1 which said step of determiningthe unbalance in pressure distribution comprises steps of detectingpressure rise in one compressor and the total pressure rise in thecompressors, said setting angle of the adjustable stator blades beingadjusted in accordance with the ratio of said pressure rise in said onecompressor to said total pressure rise.

5. A method in accordance with claim 1 in which the speed of thecompressors is controlled in accordance with the output pressurethereof.

6. A method in accordance with claim 1 in which the speed of thecompressors is controlled in accordance with air flow.

1. A method of controlling multi-casing variable speed compressors at least one of which is of axial compressor type and has stator blades which are adjustable in setting angle, said method comprising steps of determining the unbalance in pressure distribution, adjusting the setting angle of the stator blades so as to compensate unbalance in pressure distribution between the compressors, and controlling the speed of the compressors for adjusting the total pressure rise or air flow.
 2. A method in accordance with claim 1 in which said compressors include a high pressure compressor and a low pressure compressor, said high pressure compressor having adjustable stator blades.
 3. A method in accordance with claim 1 in which said compressors include a high pressure compressor stage and a low pressure compressor stage, said low pressure compressor having adjustable stator blades.
 4. A method in accordance with claim 1 which said step of determining the unbalance in pressure distribution comprises steps of detecting pressure rise in one compressor and the total pressure rise in the compressors, said setting angle of the adjustable stator blades being adjusted in accordance with the ratio of said pressure rise in said one compressor to said total pressure rise.
 5. A method in accordance with claim 1 in which the speed of the compressors is controlled in accordance with the output pressure thereof.
 6. A method in accordance with claim 1 in which the speed of the compressors is controlled in accordance with air flow. 